U.S. Pat. No. 4,735,591 issued Apr. 5, 1988 discloses and claims a method of making a long life high current density cathode from tungsten and iridium powders using a barium iridiate as the impregnant. More particularly, according to that patent, a mixture of tungsten and iridium powders is processed with an activator into a porous billet and the porous billet then impregnated with a barium iridiate such as Ba Ir O or Ba Ir O in a dry hydrogen furnace at a temperature at which the impregnant melts. Though the cathode prepared by the method of U.S. Pat. No. 4,735,591 exhibits an improved operating life, the method of the patent is not entirely satisfactory in that the barium iridiate impregnant is difficult to prepare in the pure state. This is because the iridium oxide, Ir.sub.2 O.sub.3 vaporizes off before the barium oxide, BaO can form.
That is, in preparing the barium iridiate impregnant, one must first heat barium carbonate, BaCO.sub.3 to 1450.degree. C. to convert the barium carbonate to the oxide according to the reaction: EQU BaCO.sub.3 .fwdarw.BaO+CO.sub.2
One must be careful to remove the BaO from the furnace and place it immediately into an inert atmosphere since BaO reacts with CO.sub.2 and H.sub.2 O from the air to form barium carbonate and barium hydroxide according to the reactions: EQU BaO+CO.sub.2 .fwdarw.BaCO.sub.2 and EQU BaO+H.sub.2 O.fwdarw.Ba(OH).sub.2
As the barium oxide is being weighed, the latter two chemical reactions occur, thus diminishing the amount of available BaO. The Ir.sub.2 O.sub.3 is mixed into the BaO in a molar ratio of 3BaO/1Ir, and the material is then placed into a hydrogen furnace.
In summary, one of the disadvantages in the forming of the barium iridiate impregnant is the competition of the carbon dioxide and water from the atmosphere to form barium carbonate and barium hydroxide. Another disadvantage is that Ir.sub.2 O.sub.3 loses oxygen at 400.degree. C. which tends to push the sample out of the impregnation cup. A third disadvantage is that the residual barium carbonate and barium hydroxide formed in the reaction must be heated to 1450.degree. C. to convert the barium carbonate and barium hydroxide into the barium oxide again.